As is known by those skilled in the art, the above-outlined structures are used extensively to drive motors and inductive loads in general, using either bipolar or the field-effect type transistors. The half-bridge structure, mainly using field-effect transistors, is specifically employed to drive brushless and stepper motors, and also to transmit high-voltage logic signals. In fact, wherever the implementing technology so allows, VDMOS (Vertical Double Diffused MOS) components are used as field-effect transistors on account of the many advantages that they afford over bipolar components.
The two field-effect transistors which form a half-bridge are connected in series with each other between the two terminals of a voltage supply generator, in other words between the "power supply" and the ground potential, and are alternately driven into conduction by control circuit means coupled to their gate terminals.
The voltage amplitude of the output signal from the linking node between the two transistors is both dependent on the output current and the saturation resistance of the conducting transistor.
To minimize the voltage drop between the power supply and the output, it is necessary to impose on the upper transistor, during the conduction phase, a gate voltage higher than the source voltage (usually about 10 V), to optimize the saturation resistance.
By methods well known to those skilled in the art, e.g. through the use of a voltage multiplier, a predetermined optimum voltage amount is established, which is higher than the supply voltage, and this higher voltage is applied, during the conduction phase, to the gate terminal of the upper transistor to provide optimum gate/source voltage independently of the supply voltage.
It is common practice to protect the upper transistor against unavoidable voltage peaks due to the turning on/off of the transistor--which are even higher than this optimum voltage--by means of two Zener diodes connected between the source and the drain, reversed one from the other.
The above problem does not exist with the lower transistor, which is automatically protected because it is driven by internally generated voltages. In addition, the source terminal of the lower transistor is always maintained at a reference potential, whereas the source terminal potential of the upper transistor may vary between the two potential levels of the voltage supply generator, that is to say between the "power supply" and the ground potential.
A conventional method of turning off the upper field-effect transistor is to pull the gate terminal of such transistor to ground via a depletion current generator connected in series with a switch to be closed during the off phase. The current generator may be connected to a reference voltage, and the switch may be another field-effect transistor driven to switch over.
An improvement on this arrangement has been provided in practice by connecting the gate terminal of the upper field-effect transistor to a depletion current generator, again via a switch, not directly but with the interposition of a PNP type of bipolar transistor, the emitter and collector terminals of said bipolar transistor being directly connected to the gate and source terminals respectively of the field-effect transistor and its base terminal being coupled to the ground depletion current generator via the switch.
The advantages of this arrangement for turning off the field-effect transistor are that the transistor's own gate capacitance discharge current can be dissipated directly to the load, and that upon turning back on, the gate terminal will already be at the same potential as the source terminal, which does not need to be the ground potential. Furthermore, the current from the depletion current generator is lowered by a factor equal to the PNP transistor gain.
Disadvantages inherent to this arrangement include the frequency limitations and increased integration area requirements of the bipolar transistor. Moreover, if a higher voltage than the voltage drop on the base/collector junction of the PNP transistor is imposed at the output, a pull from the output occurs through said junction.
Specifically in the instance of a half-bridge structure--but not in that of the High Side Driver--with the circuit configurations described herein used as a line driver, or for driving loads connected to the positive supply line, a further problem arises. When the upper transistor is turned off through the depletion current generator and the lower transistor turned back on, the potentials at the source and gate terminals of the upper stage transistor may be enough to turn this transistor back on, thereby an uncontrolled current would be produced, commonly referred in the art to as "cross-current".